The invention concerns a process and apparatus for controlling the temperature of the fluidized bed in a thermal reaction furnace, also known as an incinerator, used for the incineration of liquid, solid or gaseous substances while feeding fuel into the fluidized bed.
When incinerating liquid, solid or gaseous substances, while feeding additional fuel into the fluidized bed of a thermal reaction furnace, there will be a heterogeneous mixture of highly heat-absorbing matter present in the furnace. In order not to interrupt the thermal process, it is necessary to ensure, by employing temperature control, that ignition temperature prevails at one point of the mixture at least. To initiate the thermal process, i.e. when starting up the thermal reaction furnace, it is necessary to heat the fluidized bed to a temperature that is equal to, or higher than, the ignition temperature of the fuel.
In order to achieve complete combustion in fluidized bed incinerators, it is known to maintain a minimum temperature of about 800.degree. C within the incinerator above the fluidized bed by means of a temperature control system. The fluidized bed temperature is subject to certain fluctuations caused by the variation of the combustible material content of the matter to be incinerated. If the fluidized bed temperature should drop below the ignition temperature of the fuel, e.g. when replenishing the incinerator with sand or due to an insufficiency of combustible material, the incinerator combustion process will be disrupted.
As is known from literature, fluidized bed incinerators are relatively sensitive and their control presents certain difficulties.
In order to initiate the thermal process, i.e. to start up a fluidized bed incinerator, it is known to heat the fluidized bed to the necessary temperature by blowing hot gases from above onto the non-fluidized bed, usually consisting of sand. Conventional fluidized bed incinerators are therefore equipped with start-up burners which are arranged in a brick lined duct above the fluidized bed and which slant downwards. Normally, the start-up burner is ignited by a pilot burner with flame scanner. The sand bed is heated solely by the start-up burner which is fired with fuel oil or gas. The air throughput for the fluidized bed is adjusted to a flow rate that is just sufficient to cause fluidization, in order to minimize the loss of thermal energy as a result of heat being carried away by the fluidization air stream, while ensuring that the sand circulation rate is adequate to heat even the lowermost layers of the bed.
When employing this method for preheating the fluidized bed, a substantial quantity of the heat introduced by the start-up burner is unused because it is carried away by the air leaving the fluidized bed in an upward direction. It is not until the ignition temperature of the fuel is reached that fuel is supplied to the fuel feed nozzles. If the sand circulation rate and the supply of heat by the start-up burner should not be properly adjusted to each other, i.e. if too much heat is being supplied and the sand circulation is insufficient, the sand surface temperature will rise to a point where the sand melts. This would result in the formation of solid lumps of sand and the fluidized bed would become inhomogeneous.
Another method of starting up fluidized bed incinerators consists of installing a burner muffle for producing hot gas below the supporting tray for the fluidized bed or outside the incinerator and passing the hot gases through the fluidized bed from below until the fluidized bed is at a temperature that is adequate for igniting the fuel. Both methods and the devices used therefor are expensive, cumbersome and not entirely trouble free.